1. Field of the Invention
The present invention relates to a vehicle detector that has at least one inductive loop that is used as a sensor for performing vehicle detection utilizing a fixed-frequency oscillator, a phase/amplitude controller, and an evaluation unit that permits detection of a vehicle in the detection range of the induction loop.
2. Description of the Prior Art
Vehicle detectors are employed for purposes of detecting vehicles in traffic, and may be used to detect the presence, type, and/or speed of such vehicles. Inductive loops, among other things, have long been used as sensors in these types of vehicle detectors. These inductive loops are permanently embedded in the roadway of a traffic route--in a lane-related manner, if necessary.
Vehicle detectors of this type using inductive loops as sensors exploit the effect that the loop inductance varies depending on the metallic mass of a vehicle moving in the range of the inductive loop. It has long been known that, in order to evaluate this effect, the inductive loop must be accompanied by a modified capacitor to produce a resonant circuit, which is made to resonate by an excitation circuit. The resting frequency is defined as the frequency of this resonant circuit which arises when there is not a vehicle in the detection range of the inductive loop.
The resonant frequency changes from the resting frequency when the loop inductance changes, caused by a vehicle. The amount of change is proportional to the mass of the detected vehicle.
In this "oscillator solution", the present resonant frequency of the resonant circuit containing the inductive loop is measured, i.e., the frequency is counted in a digital evaluation. This measured frequency is then compared to its resting frequency, and the result is output as a detector signal indicating whether a vehicle is located in the detection range of the corresponding inductivity loop and, possibly, the type of vehicle.
As taught by the documents EP-A-0 521 627, EP-A-0 523 852 and EP-A-0 523 853, e.g., this known oscillator solution is relatively sensitive to various disturbing influences, such as neighboring inductive loops or current-carrying cable which is laid in the vicinity of the inductive loops. As the cited documents prove, extensive expenditures effort and costs must be made to compensate these types of disturbing effects, which are based on magnetic coupling. The cited document EP-A-0523 852 also demonstrates that, besides the technical expenditures for eliminating disturbance, the known oscillator solution has a further fundamental systemic disadvantage; namely, it covers a broad frequency band for signal evaluation. From the aspect of signal technology, only a bandwidth of 500 Hz would be required, assuming an evaluation time of 2 ms. But the frequency band covered in the evaluation of detector signals is of necessity orders of magnitude broader.